1. Field of the Invention
The present invention relates to micro-machined valves capable of regulating flow in micro electromechanical systems (MEMS) devices and also relates to methods of making such valves.
2. Description of the Related Art
U.S. Pat. No. 6,056,269 to Johnson et al. (the Johnson ""269 patent), incorporated herein in its entirety by reference, discloses the micro-miniature valve 5 having a silicon diaphragm illustrated in FIG. 1. The valve 5 includes a diaphragm 10, a valve body 12 (made up of a seat substrate 15 and a base 17), a valve seat 20, a well or recess 30, an orifice 40, an inlet port 50, an inlet channel 60, an outlet port 70 and an outlet channel 80. The seat substrate 15 and the diaphragm 10 are made of silicon. The base 17 is made with glass.
A recess or well 30 is formed in the seat substrate 15 by a first etch step. Inside the recess or well 30 is a valve seat 20, formed by a second etch step. Further, a third etch step is sometimes used to align the features on the front of the seat substrate 15 to the features on the back of the seat substrate 15. The inlet port 50, inlet channel 60 and outlet channel 80 are formed in the seat substrate 15 via a fourth etch step. The orifice 40 and outlet port 70 are chemically etched in the interior of the valve seat 20, by a fifth etch step, such that it extends through the seat substrate 15 and connects to the inlet port 50 and the inlet channel 60. If a double-sided aligner is used, the third etch step can be eliminated. Therefore, depending upon whether a double-sided aligner is used or not, the same piece of silicon that makes up the valve 5 is etched either four or five times.
Since each photolithography, handling and etching step inherently has associated yield problems, a few wafers are lost at each step. Assuming that each step has an associated loss of 10% of the wafers, the total yield of valves 5 according to the method discussed above is 90% raised to the power of 4 or 5. Hence, only between 59 and 66% of the valves 5 manufactured by the process described above will be operational.
In operation, the valve 5 is opened and shut by the diaphragm 10. Whether the diaphragm 10 is in the open or closed position is dependant on a control pressure applied on the top surface of diaphragm 10. When the control pressure is high, the diaphragm 10 deflects onto and forms a seal with the valve seat 20, thereby closing the valve 5. However, when the pressure is reduced, the diaphragm 10 relaxes away from the valve seat 20 and opens the valve 5.
When the diaphragm 10 is relaxed away from the valve seat 20, gas or liquid can pass into the inlet port 50, through the inlet channel 60 and out of the orifice 40. Then, the gas or liquid can flow into the recess 30 and can drain through the outlet port 70, the outlet channel 80 and out of the valve 5. When the diaphragm 10 is positioned directly atop the valve seat 20, the seal created prevents gas or liquid from flowing out of the orifice 40. Hence, neither gas nor liquid can escape via the outlet channel 80 and the valve 5 is in a closed position. In some instances, the direction of flow can be reversed, making the inlet an outlet and vice versa
The diaphragm 10 can be made from a relatively thick piece of silicon bonded to the body 12 and then chemically etched from one side, leaving somewhere on the order of a 5-to 80-micron-thick diaphragm 10. However, because semiconductor-processing equipment is designed to handle wafers of certain thickness ranges, it is generally preferred to etch the diaphragm 10 before performing the bonding process. Further, a pre-etched diaphragm 10 is preferable to bonding wafers and then etching them because of wafer-to-wafer thickness variation and thickness variations at different regions on the same wafer, as discussed below.
Typically, thickness variation from one wafer to another is approximately 25 microns. This means that, in a batch of wafers specified as being 500 microns thick, some wafers may be only 487 microns thick while others may have a thickness of 512 microns. If a 500-micron etch were to be performed on all of the wafers in a batch after they were attached to a set of bodies 12, the 487-microns-thick wafers would be etched completely through while the 512-micron-thick wafers would leave 12 microns of thickness that could be used as a diaphragm 10. Hence, diaphragm 10 thickness could not be controlled precisely by standard batch manufacturing processes and the cost of manufacturing valves 5 would increase substantially.
Thickness variations at different regions on the same wafer would increase processing complexities and cost even more. Under such conditions, the diaphragm 10 could be completely etched away in some regions while too thick of a diaphragm 10 could be left in other regions. Therefore, as stated above, pre-etched diaphragms 10 are preferred.
Once a diaphragm 10 has been obtained, the fusion bonding process is used to affix the diaphragm 10 to the seat substrate 15 and to affix the seat substrate 15 to the base 17. This process requires that two very clean and flat silicon wafer surfaces be in contact with each other. Once the surfaces are in contact, the bonding process starts and a strong bond can be formed after annealing, typically in a high-temperature environment of greater than 1100xc2x0 C. The end product of the fusion bonding process can be a silicon structure that is almost monolithic. However, according to certain types of fusion bonding, one wafer can be oxidized and placed in contact with a bare silicon wafer.
Although the fusion bonding process can theoretically produce strong bonding, certain requirements and specifications have to be met. For example, many studies on wafer specifications have been performed and the need for an approximately 5 nanometer rms surface roughness is generally accepted as being required for proper bonding.
Also, extremely clean surfaces are required in order to carry out the fusion bonding process. Generally, wafer surfaces are first treated according to the well-known RCA etch/cleaning process (developed by the RCA Corp.) and immediately thereafter are bonded together. Further, the cleanliness required for fusion bonding typically necessitates the use of a class 10 or, preferably, a class 1 clean-room environment. Because such environments are expensive to maintain, the fusion bonding process is not conducive to commercial production.
The wafer surfaces must also be free of chipping. When a wafer is chipped, the chips themselves can become bare silicon surfaces. Should the chips (or particles from the chips) fall back onto either of the wafer surfaces, a gap would inevitably remain as the surfaces are placed in contact with each other. Such a gap would render wafer-to-wafer bonding impossible. Hence, having to avoid gap formation renders the manufacturing process of the valves 5 discussed above even more problematic.
If all smoothness and cleanliness conditions discussed above are not met, the silicon-to-silicon bonds holding the diaphragm 10 to the seat substrate 15 in the valve 5 either never form or are highly susceptible to delamination. Under non-ideal conditions, even when bonds form, the bonds are weak and simply inserting one""s fingernail between the two wafers causes the wafers to peel away from each other.
Assuming that ideal bonding conditions have been met, the valve 5 still has inherent design flaws that limit its use. For example, the diaphragm 10 stands a high risk of cracking during use when pressure from the top or back side of the valve 5 (opposite the body 12) is too great. Under such conditions, the diaphragm 10 is pushed against the valve seat 20 with such force that the diaphragm 10 attempts to conform to the shape of the valve seat 20. Therefore, especially at the edges of the valve seat 20, the diaphragm 10 experiences tremendous tension and the associated stress causes the diaphragm 10 to crack.
Another inherent design flaw becomes problematic when the back side of the diaphragm 10 is held at a lower pressure than the front side of the diaphragm 10. Under such conditions, the diaphragm 10 flexes away from the seat substrate 15. Since nothing prevents this flexing, the diaphragm 10 sometimes, under a sufficient reversal of pressure, flexes as much as 500 microns and cracks.
Another micro-miniature valve 6 is disclosed in the dissertation by Stephen Clark (Dissertation by Stephen Clark, Stanford University, Ph.D, E.E., May 1975, xe2x80x9cA Gas Chromatography System Fabricated On a Silicon Wafer Using Integrated Circuit Technologyxe2x80x9d, pp. 41-128. UMI Dissertation Services, Ann Arbor, Mich.), incorporated herein in its entirety by reference. The valve 6 disclosed in the Clark dissertation is illustrated in FIG. 2.
As show in FIG. 2, the Clark valve 6 includes a diaphragm 10, a base 17, a valve seat 20, an orifice 40, an inlet channel 60, an outlet port 70, an outlet channel 80, a cover 90 and a pressure inlet 310 that regulates the pressure above the diaphragm 10. The base 17, diaphragm 10 and valve seat 20 of the Clark valve 5 are made of silicon and the cover 90 is made of glass. The base 17 is bonded to the cover 90 and the diaphragm 10 is sandwiched between the valve seat 20 and the cover 90. The diaphragm 10 is also hermetically sealed to the cover 90 using glass-silicon anodic bonding.
In operation, the diaphragm 10 opens and closes the valve 6 by pressing against or relaxing away from the valve seat 20. As with the Johnson valve 5, the diaphragm 10 of the Clark valve 6 is not limited in its ability to flex away from the valve seat 20 under a back side pressure. Quite to the contrary, the diaphragm 10 is significantly detached from the cover 90, can bend backwards significantly and can therefore crack under back side pressure.
Also, like the Johnson valve 5, the diaphragm 10 of the Clark valve 6, under sufficiently large pressure, will crack as it attempts to conform to the geometry of the surface of the valve seat 20. Further, manufacturing of the base 17 alone requires four etching steps to etch the well 30, outlet channel 80, valve seat 20 and orifice 40. In other words, the Clark valve 6 operates in a manner similar to the manner in which the Johnson valve 5 operates. In addition to the disadvantages that the Johnson valve suffers, the Clark valve also requires higher precision control of etching and handling of small parts during manufacturing.
In one embodiment, a micro-valve includes a seat substrate having an outlet port and an orifice therethrough. The seat substrate includes a valve seat protruding from a well in the seat substrate, a support protruding from the well, and a diaphragm above the seat substrate.
In an alternate embodiment, a micro-valve includes a seat substrate having an outlet port and an orifice through the port. The substrate includes a diaphragm having a different material than the seat substrate and positioned above the seat substrate.
In another alternate embodiment, a method of manufacturing a first micro-valve includes steps of etching a well in a seat substrate, forming an orifice and an outlet port in the well, and anodically bonding a diaphragm to the seat substrate.